How to open an ebook on the Kindle App

You'll need to convert books to either pdf or .mobi if you want to upload them to your Kindle app as well, and even then it's not always as simple as connecting your phone to your computer. Amazon has an account-based cloud distribution system where your Amazon account effectively has an email account, and you mail yourself the book you didn't buy through Amazon. It's a mostly closed system, but it's a really good mostly closed system, and that's why so many people use it.